Instructions

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Although the word "send" is an easy way to describe it, it is not quite correct. In USB, everything is
initiated by the host. What actually happens with HID interfaces is that the USB device prepares a report
and makes it available to the host. Then, on a regular interval, the host polls the device to see if it has any
reports ready. In the Descriptor Tool, this particular interface was set to have the fastest possible polling
interval: 1 ms.
After receiving a report indicating a keystroke, the host assumes the key is held down until a report is later
sent to indicate its release. Because of this, the demo application quickly follows every key-press report
with a key-release report.
So when a LaunchPad pushbutton press occurs, it sets a flag and wakes main(), had it been sleeping in
LPM0. Execution eventually checks the flags associated with the buttons. If the button had been pressed,
it calls prepSendingStr() to fetch the target string from the file associated with that button.
// Handle a press of button 1, if it happened
if (bButton1Pressed && !charLeftToSend)
{
prepSendingStr("0:Button1.txt");
bButton1Pressed = FALSE;
}
It uses high-level FatFs calls to do this – to mount the volume, open it, read it, and close it. When the
string has been obtained, main() assigns the length of that string to charLeftToSend. While this variable is
non-zero, it means there are still characters left to transmit to the host.
Later in main(), code evaluates charLeftToSend, and also checks whether a USB report is still waiting to
be fetched from the host. If characters still need to be sent, and if the USB HID interface is available, the
report is prepared, and USBHID_sendReport() is called to "send" it. A flag bKeyDirIsDown is used to
alternate between down-presses and up-presses, to ensure every down-press is followed by an up-press.
if (bUsbSendComplete && charLeftToSend)
{
if(bKeyDirIsDown) // Will this be a down-press?
{
KB_addKeypressToReport(btnStr[btnStrLen-charLeftToSend]);
bKeyDirIsDown = FALSE;
}
else // Or will it be an up-press?
{
KB_addKeyReleaseToReport(btnStr[btnStrLen-charLeftToSend]);
bKeyDirIsDown = TRUE;
charLeftToSend--;
}
bUsbSendComplete = FALSE;
USBHID_sendReport(KB_getReportPtr(), HID0_INTFNUM);
}
USBHID_sendReport() copies the report to the USB endpoint buffer, making it available to the host.
HID0_INTFNUM is a value that references this particular HID interface; if additional HID interfaces had
been created within this device, this parameter is how code could access them separately. The Descriptor
Tool defines an INTFNUM constant for every interface it creates, stored in descriptors.h.
When the host gets around to fetching the report, a USBHID_handleSendCompleted() event is generated.
BYTE USBHID_handleSendCompleted (BYTE intfNum)
{
bUsbSendComplete = TRUE;
return (TRUE); // Returning TRUE wakes the main loop, if it had been
} // sleeping.
This is one of the USB event handlers in usbEventHandling.c. These handlers are defined by the API, and
the developer can insert code that should execute when those events occur. In this application, the
bUsbSendComplete flag is set to TRUE in the handler, and the handler returns TRUE, which wakes
main() if it had been sleeping at the LPM0 entry. This allows main() to send the next character, if one is
waiting to be sent.
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SLAU533A–September 2013–Revised January 2014 MSP430F5529 LaunchPad™ Development Tool (MSP ‑ EXP430F5529LP)
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